No it is not. Her first appearance is in
this strip, second last panel. It's not a last panel joke. In fact, it's not a joke at all; it's an expanatory panel, just like the one immediately above it.
I should note that not all last panels have last-panel jokes. First of all, there has to be a joke in it, and some don't have any. Secondly, even when there is a joke, it may not be what should be condsidered a last-panel joke. There's usually some kind of fourth-wall breaking in them, and not all jokes in last panels do that.
Kish above gave a more explicit name for them: "fourth-wall-breaking last-panel punchline". Unfortunately, that's too much of a mouthful for everyday conversation.
Here's some examples of last-panel jokes:
- 918 anyone think Director Lee's inbox will to be an important plot element in the future?
- 206 and 389 think any of the stuff Elan and Thog have in those panels will ever show up again?
- 748 Belkar's been knocked back to Basic D&D, yet strangely enough, it doesn't seem to be permanent, or even ever mentioned again.